A serine protease has been identified that initiates coagulation by directly activating factor X and is inhibited by diisopropylfluorophosphate (DFP). This protease is extracted from tumor tissue and is present in serum free medium from transformed and malignant cells in culture but is not found associated with most of the normal tissues or cultured cells evaluated. This protease has been purified to near homogeniety. The proposed study will complete the purification and determine some of the physicochemical characteristics of the protein. Antibodies will be developed to the protein and some of the immunochemical features will be determined and an immunoassay will be established. The levels of this protease will be measured in serum free medium of cultured human malignant cells and in blood from animals and patients with cancer.